In one form of a fluid pressure control valve, a flow control element, such as a spool or valve member, is movably positioned in a valve chamber between first and second valve positions for selectively fluidically coupling valve ports. The spool member may be directly actuated by a movable armature or plunger. A solenoid controllably positions the plunger, which results in movement of the spool from a neutral position to an actuated position. Typically, a coil spring is used for biasing the spool to the neutral position. Energization of the solenoid coil produces a magnetic force acting on the plunger which is related to a gap between the plunger and a stop. This relationship is represented by a curve in which force is generally inversely proportional to gap. With an on/off type valve, the magnetic force exceeds the opposing spring and flow forces to provide continuous movement of the plunger to a fully actuated position.
With a proportional type valve, it is necessary to control the plunger to stop at intermediate positions. An example of such a proportional valve is shown in Kolchinsky, U.S. Pat. No. 4,790,345. This patent shows a valve housing including a sleeve having first and second tube portions connected by a non-magnetic bridge. The mating surfaces of the bridge and tube portions are conical. This construction changes magnetic behavior as by modifying the above-described curve to provide constant force during the portion of the stroke at which the plunger is at an axial position corresponding to position of the bridge. As a result, the plunger moves until the magnetic force is balanced with the spring and flow forces so that the plunger stops at an intermediate position. As is known, the magnetic force depends on voltage applied to the coil. Thus, by varying coil voltage the stop position varies. Thus, movement of the spool can be controlled to regulate flow by varying input voltage.
In the case of a pressure control valve, a similar tube is used. However, the force of the pressure differential over the cross section of the spool cross section opposes the magnetic force. Therefore, the larger the diameter of the spool, the larger the required magnetic force.
While such typical proportional valves are satisfactory, they are also expensive to produce because of the manufacturing steps required in constructing the tube with the bridge.
Clark, U.S. Pat. No. 4,604,600, discloses an alternative construction having a one piece non-magnetic tube. This valve has a conical lip in either the plunger or stop and not the tube wall. Particularly, this design uses a conical lip on either an outer wall of the stop or the plunger or on an inner recess for a cylindrical nose of either the stop or the plunger. The nose is received in a cylindrical recess on the other of said parts. In all cases, there is a cylindrical joint between the plunger and stop. This construction is believed to provide a poor performance compared to the structures using the magnetic bridge.
It is also desirable to increase regulated pressure capacity for a given solenoid force without decreasing spool diameter. Decreasing spool diameter lowers pressure capacity.
Finally, it is desirable to decrease frictional forces relating to axial sliding movement of the plunger and valve member.
The present invention overcomes one or more of the problems discussed above.